The present invention relates to a cathode ray tube including a channel plate electron multiplier and more particularly to the production of images in two or more colors in such tubes.
British Patent Specification Nos. 1446774, 1452554 and 2143077A disclose a method of producing color images in a cathode ray tube including a channel plate electron multiplier in which the electron beam emerging from a channel is subjected to an electrostatic field whereby the beam is formed into a dot or a circle of variable diameter. The phosphor screen comprises a dot of one color surrounded by a ring of another color which is surrounded by a third color. In order to produce the necessary electrostatic fields to cause the electron beam to impinge upon the relevant phosphor, one or more apertured electrodes are mounted on the extractor electrode of the channel plate electron multiplier. The apertures in the or each electrode are circular having the same pitch as the apertures in the electron multiplier. By adjusting the focusing voltages applied to the extractor electrode and the or each electrode carried thereby, the cross sectional shape of the beam can be changed from cylindrical to annular. The color selection electrode or electrodes consist of etched components which are easily manufactured, for example from mild steel which may be used in forming the half dynodes of the electron multiplier. Although this method of color selection is viable, it has the disadvantages that high and complex switching voltages are required and also very accurate alignment is needed between the electron multiplier and the fluorescent screen.
British Patent Specification Nos. 2124017A and 1458909 disclose a method of color selection in which elongate deflection electrodes are disposed between the columns of channels in the channel plate electron multiplier. In particular when pairs of interdigitated electrodes are disposed between the columns of channels on the output side of the electron multiplier (Specification No. 2124017A) the quality of the colored image produced is of a very high standard. In operation the electrons emerging from the channels are subjected to an asymmetric lens field which elongates the electron beams in the column direction whilst simultaneously deflecting them to a series of color stripes on the screen. This method has acceptable switching voltages and much less stringent requirements for alignment between the multiplier and the screen as compared to the dots and rings method. However a series of pairs of deflection electrodes positioned between each column of channels is required and this arrangement is difficult to realize in practice.